The measurement of small current signals on top of or after high current peaks is a challenging experimental task. Usually three different types of measurement techniques are employed to solve this problem:
1) The first measurement principle is based on the integration of an additional resistance into the circuit, a so called shunt resistor, such that the current which is to be measured (main current) flows through it. The current is then directly obtained from the voltage drop across this additional resistance.
However, there are difficulties associated with this measurement technique. On one hand the resistance has to be chosen such that it does not influence the circuit and thus has to be much smaller than all the other resistances. Also, during the peak current it is not allowed to heat up as this would generally change its characteristics. With this, the voltage drop becomes very small for small currents after the peak and the accuracy of the measurement decreases. Small variations on top of a large current can also not be resolved for the same reason, as they only cause small variations in the voltage drop.
2) The second measurement technique relies on the detection of the change in the magnetic field generated by the change in the current. Different techniques can be applied for this purpose. The most widely used technique bases on a coil that is placed around the current carrying cable. Changes in the magnetic field induce a voltage in the coil which can be directly measured. This signal has to be integrated in order to obtain the current signal itself.
The integration adds up the errors of each individual measurement resulting in an offset of the current signal. Hence, after a large current peak small currents cannot be measured accurately due to the offset in the signal. On top of a large current, small but fast changes can be measured with this technique. However, slow changes, e.g. drifts or variations of peak voltage, cannot be measured accurately. In order to reach high accuracy with this technique very small changes of the magnetic field have to be detected with a high bandwidth. This can either be realized by using a large number of windings for the coil which limits the bandwidth to low values only or by adding a ferromagnetic core to the coil in order to enhance the magnetic field and thus the signal. The difficulty with the ferromagnetic core is that high magnetic fields will magnetize the material and will thus lead to a distortion of the measured signal and to additional offsets due to hysteresis effects.
3) A far more elegant way is to measure the magnetic field directly instead of its change. With this, the problem with the integration offsets is avoided. However, as the magnetic fields generated by small current are small, the sensitivity is usually increased by employing magnetic materials. Thus, still offsets will be present due to hysteresis effects in these materials.
A further approach for the measurement of currents is by using magnetic field sensors with high sensitivity, as for example magnetoresistive sensor elements. As for example disclosed in U.S. Pat. No. 5,708,407 it is possible to use a circular current sensor based on a magnetoresistive material through which the conductor of the currents to be measured is guided. WO 2006/042839 discloses an improvement of such a device in that hysteresis or offset problems are said to be overcome by applying a pulsed additional magnetic field in the region of the sensor prior to the actual measurement process.